In RF digital communication systems, the bandwidth of a channel is generally preset in accordance with the permitted frequency spectrum rather than chosen on the basis of the data characteristics. Once the channel bandwidth is known, the data rate and the shape of the digital signal may be selected so that the permitted frequency spectrum is not exceeded. It has also been necessary to avoid nonlinearity in transmission which could cause the RF spectrum of the signal to spill over the assigned channel bandwidth.
Techniques have been devised to optimize signal reception by controlling the bandwidth of a receiver. In some arrangements, the bandwidth is varied in accordance with the signal to noise ratio. U.S. Pat. No. 4,189,755 issued to M. S. Balbes et al, Feb. 19, 1980, discloses a television receiver threshold extension system in which the received video signal and noise are sampled and signal representative of the signal to noise ratio is formed. When the received signal drops to an objectionable level, the bandwidth of the receiver is narrowed to reduce the signal noise.
U.S. Pat. No. 4,792,993 issued to John Y. Ma, Dec. 20, 1988, discloses a receiver system having automatic bandwidth adjustment for signals with varying bandwidths. A bandwidth detection circuit indicative of the bandwidth generates a filter bandwidth control signal which selects one of a plurality of bandpass filters to accommodate the incoming signal.
Some arrangements vary receiver bandwidth in accordance with amplitude of the received signal. U.S. Pat. No. 3,904,968 issued to B. M. Brinegar, Sept. 9, 1975, for example, discloses a signal sensitive switching circuit that allow normal operation of a receiver when the received signal is greater than a predetermined amplitude and inserts a narrow band filter to reduce the effects of noise when the received signal is less than the predetermined amplitude.
With respect to data signals, U.S. Pat. No. 4,045,740 issued to K. L. Baker, Aug. 30, 1977, discloses a method of optimizing the bandwidth of a radio receiver to accommodate signals having widely varying data rates wherein the optimum intermediate frequency for a given data rate is determined and a filter having a continuously tunable bandwidth is adjusted accordingly.
As is well known in the art, signal detection for data occurs between transitions of the demodulated data waveform. The receiver bandwidth, however, is generally set to accommodate the wide frequency spectrum of the transitions and may be varied as aforementioned in accordance with the amplitude or signal to noise ratio of the received signal. As a result of the wide bandwidth to accommodate transitions, the noise remains at a high at the detection instants. This noise due to transition determined bandwidth results in lower detection accuracy. It is an object of the invention to provide an improved digital signal transmission arrangement having more accurate detection without limiting the data transition rate.